1. Field of the Invention
This invention broadly relates to a process for producing useful chemicals, and especially formaldehyde, from methane (CH4) and hydrogen sulfide (H2S), and especially from a gas stream containing a mixture of CH4 and H2S. More particularly, this invention provides a method wherein hydrogen sulfide, generally separated from a gas stream containing methane and hydrogen sulfide, is combined with a carbon oxide, wherein the carbon oxide is selected from carbon monoxide (CO), carbon dioxide (CO2) and mixtures thereof, and the combined gas stream is passed in contact with a catalyst comprising a supported metal oxide of a metal selected from the group consisting of vanadium (V), niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium (Re), titanium (Ti), tungsten (W), manganese (Mn), tantalum (Ta) and mixtures thereof to convert said carbon oxide and hydrogen sulfide mixture to methyl mercaptans, (methanethiol(CH3SH) and dimethyl sulfide (CH3SCH3)). The methyl mercaptans can be used as a starting material for making additional products and preferably are then passed in contact with a catalyst comprising certain supported metal oxides or certain bulk metal oxides in the presence of an oxidizing agent and for a time sufficient to convert at least a portion of the methyl mercaptans to formaldehyde (CH2O) and sulfur dioxide (SO2). The carbon oxide used to react with the hydrogen sulfide is produced by the partial oxidation of methane in the presence of oxygen over a metal partial oxidation catalyst that promotes the partial oxidation of methane to carbon oxides, preferably CO, and hydrogen (H2).
2. Description of Related Art
Natural gas recovered from geological formations often contains hydrogen sulfide as an undesired impurity in concentrations of 10-30%. The hydrogen sulfide is typically separated from the methane and often is converted to elemental sulfur via the Claus Process. In the Claus Process, a first portion of the separated hydrogen sulfide is converted (oxidized) to sulfur dioxide (SO2) and the remaining portion of the hydrogen sulfide is reacted with the sulfur dioxide in the presence of a suitable catalyst to produce water and elemental sulfur. The so-produced sulfur represents a low value-added, commodity product; while the de-sulfurized methane typically is distributed for industrial and personal uses, such as for home heating. An alternative way for producing and using the hydrogen sulfide would significantly improve the economies of sour natural gas recovery and processing.
Ratcliffe et al., U.S. Pat. Nos. 4,570,020 and 4,668,825 describe catalytic processes for producing methanthiol (CH3SH) from a gaseous feed comprising a mixture of carbon monoxide (CO) and hydrogen sulfide (H2S). According to the ""020 patent, the gaseous mixture is contacted, at a temperature of at least about 225xc2x0 C. with a catalyst comprising a metal oxide of a metal selected from the group consisting of vanadium (V), niobium (Nb), and tantalum (Ta) and mixtures thereof supported as an oxide layer on titania. In the ""825 patent, the gaseous mixture is contacted with a rutile titania catalyst under similar conditions. The methanethiol is disclosed as being useful as an odorant or tracer for natural gas and as a raw material for making methionine, fungicides and jet fuel additives.
The art has also identified methyl mercaptans, such as methanethiol (CH3SH) and dimethyl sulfide (CH3SCH3), as hazardous pollutants, and has suggested a variety of ways for their destruction. Noncatalytic gas phase oxidation of such reduced sulfur compounds has been shown to produce primarily sulfur oxide and carbon oxide products. A. Turk et al., Envir. Sci. Technol 23:1242-1245 (1989). Investigators have observed that oxidation in the presence of single crystal metal surfaces (Mo, Ni, Fe, Cu) results in the formation of methane and ethane, nonselective decomposition to atomic carbon, gaseous hydrogen and the deposition of atomic sulfur on the metal surface via a stoichiometric reaction (See Wiegand et al., Surface Science, 279(1992): 105-112). Oxidation of higher mercaptans, e.g., propanethiol on oxygen-covered single crystal metal surfaces (Rh), produced acetone via a stoichiometric reaction at low selectivity and accompanied by sulfur deposition on the metal surface (See Bol et al., J. Am. Chem. Soc., 117(1995): 5351-5258). The deposition of sulfur on the metal surface obviously precludes continuous operation.
The art also has disclosed using catalysts comprising a two-dimensional metal oxide overlayer on titania and silica supports, e.g., vanadia on titania, for catalytically reducing NOx by ammonia to N2 and H2O in the presence of sulfur oxides. Bosch et al., Catal. Today 2:369 et seq. (1988). Thus, such catalysts are known to be resistant to poisoning by sulfur oxides. It also is known that such catalysts, as well as certain bulk metal oxides catalysts, can be used to oxidize methanol to formaldehyde selectively. Busca et al, J. Phys. Chem. 91:5263 et seq. (1987).
Applicant discovered that supported metal oxide catalysts, such as vanadia on titania, can be used to oxidize methyl mercaptans, such as methanethiol (CH3SH) and dimethyl sulfide (CH3SCH3), selectively to formaldehyde in a continuous, heterogenous catalytic process without being poisoned by the reduced sulfur. On the basis of that discovery, applicant now has envisioned the present novel process for converting methane, particularly methane in a sour natural gas stream, to formaldehyde.